The basic metal-oxide-semiconductor field effect transistor (MOSFET) typically includes a gate oxide grown on a silicon substrate by thermal oxidation. Generally, the performance of the MOSFET is inversely proportional to the thickness of the gate oxide. Efforts to enhance performance and reduce power consumption have driven the thickness of the gate oxide to well below 100 Angstroms. However, such scaling introduces new problems into device fabrication and performance. For example, leakage currents may increase due to reduced gate oxide resistance to hot carrier injection. The rate of thermal oxidation can also be higher than practically manageable during formation of ultra-thin gate oxides.
Accordingly, manufacturers have begun incorporating nitrogen into gate oxides. For example, the surface of a silicon wafer may be enriched with nitrogen by implanting nitrogen atoms into the silicon surface to facilitate the growth of a nitrided oxide on the enriched wafer surface. Nitrogen may also be infused into the gate oxide by remote plasma nitridation. Such processes are introduced in U.S. Pat. No. 6,362,085 to Yu, et al., commonly assigned herewith, and incorporated in its entirety herein. This two step nitrogen enrichment process increases the dielectric constant of the gate oxide, not only decreasing its effective thickness with respect to gate capacitance, but also reducing leakage currents by increasing gate oxide resistance to hot carrier injection. In addition, because the initial silicon surface is nitrogen rich, the thermal oxidation rate is reduced, thereby improving process control by rendering the oxidation time and temperature more manageable.
Obviously, the nitrogen content of such nitrided oxides has significant impact on the subsequent processing and electrical performance of the resulting transistors. The detection of the nitrogen content of nitrided oxides typically employs secondary-ion mass spectroscopy (SIMS) or x-ray photoelectroscopy (XPS). However, SIMS is a destructive procedure, such that employing SIMS is limited to test and quality control wafers. Consequently, because SIMS is a destructive procedure and cannot be performed on production wafers, the determination of the nitrogen content of nitrided gate oxides formed on production wafers is relegated to speculation and estimates. Therefore, the true nitrogen content of nitrided gate oxides formed on production wafers may vary from design values due to uncontrolled process variations and fluctuations, possibly limiting their performance. Moreover, although XPS is non-destructive and can be used as an inline monitoring method, it is very expensive and requires air exposure, which can impact the electrical properties of the gate oxide being tested, particularly for ultra-thin oxides used in the 90-nm device technology and beyond.
Accordingly, what is needed in the art is a method for determining and monitoring nitrogen content of gate oxides that addresses the problems discussed above.